In recent years, as an environmental problem has become a matter of great concern, a honeycomb filter for removing fine particles (particulates) in exhaust gas of a diesel engine or the like has attracted attention.
As shown in FIGS. 4(a) and 4(b), the honeycomb filter has a main construction such that through holes 14a and 14b opening on both end surfaces of a honeycomb-shaped filter base 12 formed of porous ceramics are clogged in a checker flag form on one end surface, and the through holes 14b different from the through holes 14a being clogged on one end surface are clogged on the other end surface in a checker flag form. Exhaust gas 17 introduced from either one of end surfaces is forcedly caused to pass through a partition wall 13 between the through holes, by which fine particles (particulates) in the exhaust gas 17 are collected and removed.
Conventionally, there has been widely used a honeycomb filter in which ceramic slurry is pressed into the through holes 14 in the filter base 12 formed of porous ceramics and thereafter is fired, by which a filler material 11 is embedded in the through holes 14 in the filter base 12. In this case, the filler material 11 is not fixed to the partition wall 13 between the through holes by melting-reaction, but slurry is caused to intrude into pores in the partition wall 13 by pressing and is fired, and resultantly the filler material 11 is fixed by, so to say, a mechanism of fitting.
In the conventional honeycomb filter, however, a difference in coefficient of thermal expansion between the filler material 11 and the filter base 12 is not especially considered. Therefore, there arises a problem in that when the coefficient of thermal expansion of the filler material 11 is higher than that of the filter base 12, the filler material 11 intruding into the pore in the partition wall 13 produces cracks on the partition wall 13 due to thermal expansion, and when the coefficient of thermal expansion of the filler material 11 is lower than that of the filter base 12, the filler material 11 comes off.
To solve this problem, there has been proposed a honeycomb filter in which by a specific combination of ceramic materials of the filter base 12 and the filler material 11, the difference in coefficient of thermal expansion between them at temperatures of 40 to 800° C. is made 3.5×10−6/° C. or less (JP-B-2-53083).
In this honeycomb filter, however, a part of the partition wall 13 of the filter base 12 is covered by the filler material 11 because of its construction, so that the exhaust gas 17 cannot pass through the portion covered by the filler material 11, which presents a problem of decreased filter function.
Also, conventionally, in order to prevent the filler material 11 from coming off by increasing the fixing area between the filter base 12 and the filler material 11, the filler material 11 has been embedded in the through hole 14 in a thickness of about 10 to 15 mm, which presents a problem in that the pressure loss is high and hence the engine output decreases.
To solve these problems, there has been proposed a honeycomb filter in which, as shown in FIG. 5, sealing plates 21, in which the through holes 15 in the filter base 12 are clogged in a checker flag form on one end surface, and through holes different from the through holes being clogged on said one end surface are clogged in a checker flag form on the other end surface, are fixed on both end surfaces of the filter body 12 (JP-A-55-114324).
In this honeycomb filter, because the sealing plate 21 is fixed on the end surface of the filter base 12, a part of the partition wall of the filter base 12 is not covered by the sealing plate 21, so that the filter function can be increased, and also cracks can be prevented from being produced on the partition wall.
However, in this honeycomb filter, since a difference in coefficient of thermal expansion due to the difference between ceramic materials of the sealing plate 21 and the filter base 12 is not considered at all, there arises, in practical use, a problem of thermal shock resistance, such that exhaust gas leaks due to separation or coming-off of the sealing plate 21.
Also, in the case of this honeycomb filter, since a thickness of the sealing plate 21 is not considered at all, the ceramic crystals are not oriented sufficiently, depending on the thickness of the sealing plate 21 when the sealing plate 21 in which ceramic crystals are oriented by extrusion molding is formed. As a consequence, in fact, since the difference in coefficient of thermal expansion between the sealing plate 21 and the filter base 12 increases, in the case of the honeycomb filter in which this sealing plate 21 is fixed to the filter base 12, exhaust gas may leak due to separation or coming-off of the sealing plate 21 during its actual use at a high temperature. Also, the conventional problem of decreased engine output caused by increased pressure loss cannot be solved.
On the other hand, the honeycomb filter has conventionally manufactured by a method in which after slurry consisting of ceramics is pressed into the filter base formed by firing a honeycomb-shaped dried body, or after a sheet-shaped molded body before firing is pressed, firing is performed.
In this conventional manufacturing method, however, the firing process consists of two steps: a step in which the honeycomb-shaped dried body is fired to form the filter base and a step in which after slurry is pressed in or after the sheet-shaped molded body is pressed, firing is performed to form a honeycomb filter. This is a main cause of high cost of honeycomb filter, and is a great hindrance to widespread use of honeycomb filter.
Also, when the sheet-shaped molded body is pressed and fired to form a honeycomb filter, the firing shrinkage differs between the filter base already fired and the sheet-shaped molded body before firing, so that even if the through holes in them are aligned with each other at the time of pressing, a shift may occur after firing.
Further, in the manufacturing method in which firing is performed after the sheet-shaped molded body is pressed on the filter body, the bonding strength between the filter base and the sealing member is not necessarily sufficient.
The present invention has been achieved to solve the above problems, and accordingly a first object thereof is to provide a honeycomb filter which has high filtration efficiency and high thermal shock resistance, and also has high bonding strength between a sealing member and a filter base and a low pressure loss.
Also, a second object of the present invention is to provide a method for manufacturing a honeycomb filter, which can manufacture, at a very low cost and precisely, a honeycomb filter which has high filtration efficiency and high thermal shock resistance, and also has high bonding strength between a sealing member and a filter base and a low pressure loss.